The present invention relates to ion-beam technology, in particular, to electron-cyclotron resonance type ion beam sources for use in ion implanters. Implanters of this kind find application for ion implantation in the manufacture of electronic devices such as LSI and VLSI semiconductor circuits.
An ion implanter is a device, which is used for material processing in industrial manufacture. Major application of them is concentrated on semiconductor device fabrication, especially on modifying electrical properties of semiconductor materials by ion implantation, and in particular for implantation of boron and phosphorus ions into silicon. An ion source constitutes an important part of the aforementioned implanter, and its operation determines the efficiency, reliability, and performance characteristics of the implanter. The ion-beam source used in the implanter ionizes neutral molecules and accelerates the obtained ions from hundreds of eV to the required energy level of several hundred KeV. The ions are then formed into a uniform beam of a given shape and extension.
Until recently, however, the majority of ion-beam sources used in implanters for semiconductor industry were based on cumbersome, complicated, and expensive ion acceleration techniques. Examples of such technique are described, e.g., by V. V. Simonov, et al. in xe2x80x9cOborudovanie Ionnoi Implantatsiixe2x80x9d (Ion Implantation Equipment), Moscow, xe2x80x9cRadio I Svyazxe2x80x9d Publishers, 1988, pp. 35-38.
In 80""s, ion-beam sources using large plasma volumes with simplified methods for the formation of ion beams, e.g., with the use of ion-plasma optics, came into use. Almost all of them consisted of two main functional units, i.e., a gas-discharge chamber for generating plasma used as an ion emitter and an ion-optical system for extracting ions from the plasma, accelerating the extracted ion, and forming a directional ion beam. The working medium used as a material for implantation was a gaseous substance supplied to the discharge chamber or a solid substance, e.g., a solid material sputterable into the plasma volume.
Since requirements of operation conditions vary from one application to another in a very wide range, it is difficult to create a universal ion source that could satisfy all these conditions at the same time. The plasma-type ion sources have found wide application in ion implanters due to high reliability and operation performance. Depending on methods of plasma generation, these ion sources can be roughly classified as high-frequency and microwave ion sources, cold-cathode type ion sources, plasma sources with hot cathodes, Penning-discharge type ion sources with hot cathodes, quasi-magnetron sources, low-pressure ion sources with arc discharge, etc. Given below is a short description of the aforementioned ion sources which find practical application.
High-frequency and microwave ion sources are based on the use of high-frequency or microwave energy for generating plasma. Of this group of ion sources, so called electron cyclotron resonance sources (hereinafter referred to as ECR sources) have found a very wide practical application. In these sources, a phenomenon of electron cyclotron resonance (ECR) is used for increasing effective concentration of electrons in plasma and thus makes it possible to generate plasma of high density. ECR is resonance of electrons in a magnetic field on a predetermined frequency, such as 2.45 GHz for the magnetic field of 0.0875 T intensity. ECR sources can be used as ion emitters with extraction of ions from both the end faces and sides of the gas-discharge chamber. Since the present invention pertains to implanters with ECR sources, they will be described later in more detail.
Cold-cathode type ion sources are sources with cold cathodes, which generate plasma in a glow discharge due to emission of electrons into a working gas from the surfaces of cathodes, resulting in ionization of the working gas molecules. The ions formed in the working gas are accelerated toward the cathode and bombard the cathode surface causing the surface to emit secondary electrons. The plasma is formed as a result of multiple repetitions of the above process. Implanters with cold-cathode ion sources are used mainly for small and ultra-small doses of implantation. Their advantage is simplicity of construction and a relatively long service life. Disadvantages are low beam currents (not exceeding 100 xcexcA), significant fluctuations in the beam, and the limitation of using only gaseous working media.
Hot-cathode type ion sources generate an arc discharge, which is maintained by electrons emitted from the surface of a hot cathode and possessing energy exceeding the level of energy required for ionization of a gaseous working medium. Discharge occurs in a magnetic field, which is oriented parallel to the electron acceleration direction or perpendicular thereto. In the latter case the source is known as a magnetron type source. Although ion implanters utilizing such ion sources are advantageous in that they are simple in construction, are capable of generating high ion beam currents, and have a relatively stable discharge, they do not possess features required for controlling distribution of current in the resulting ion beam.
In hot-cathode ion sources with the use of a Penning discharge phenomenon for increasing concentration of electrons in plasma, extraction of ions normally occurs through a round opening in an anti-cathode (axial extraction). Ion sources of such type are known as Nilson ion sources and are used in implanters of Veeco Co. (USA) and Balzers Co. (Liechtenstein). These ion sources make it possible to utilize both gaseous and solid working media. However, with standard ion extraction energy of 30 kV, currents extracted from the Nilson-type ion sources do not exceed several hundred microamperes. Thus the implanters containing such sources inherit their disadvantages.
Quazi-magnetron ion sources, also known as Freeman ion sources, have a direct incandescence-type cathode arranged parallel to the axis of a cylindrical anode. In contrast to conventional magnetron sources, the sources of this type used for implantation have the incandescence filament offset toward the ion-emitting slit in the side surface of the cylindrical anode. Advantages of these sources, as compared to conventional hot-cathode sources, are relatively low intensity of the magnetic field (below 1 T) and weak dependence of the ion beam parameters to the parameters of discharge. The main disadvantage of quazi-magnetron ion sources is a short service life (no more than 20 hours), which is unacceptable for industrial use.
Given below is a more detailed description of ECR ion sources, which are used in ion implanters and to which the device and method of the invention pertain.
U.S. Pat. No. 5,625,195 issued to Andre Grouillet in 1997 discloses a high-energy implantation process using an ion implanter of the low-or medium-current type with an ECR ion source. In order to increase the implantation energy, this ion implanter incorporates a microwave generator with a traveling-wave tube generating an electromagnetic field with a frequency greater than or equal to 6 GHz. The initial ion source of the implanter is replaced by an electron cyclotron resonance multiply-charged ion source, including a waveguide-forming plasma cavity, whose characteristic dimension in the transverse plane of the cavity is of the same order of magnitude as the wavelength of the electromagnetic field. The microwave generator of this implanter and the plasma cavity of the multiple-charged ion source are electromagnetically coupled. A complex gaseous medium, compatible with the beam of ions desired, is admitted into the plasma cavity. The inlet flow rate of the gaseous medium is adjusted so as to maintain a residual vacuum in the plasma cavity, which is less than the pressure threshold compatible with production of multiply-charged ions. The focusing of the ion beam extracted from the plasma cavity is adjusted onto the focal point of the scanning magnet of the implanter. An ion optical system consists of three electrodes, which form an Einzel lens for adjusting the geometrical characteristics of the ion beam extracted from the cavity. This ion optics system interacts with the extraction electrode of the source, and one of the functions of this ion adjustment optics system is to focus the ion beam extracted from the cavity onto the object focal point of the scanning magnet, which allows the ion beam to pass entirely into the scanning element. More precisely, the flared extraction cone, matched to the general shape of the plasma in the plasma cavity, enables the plasma to be channeled and extracted in the form of a beam whose diameter corresponds substantially to the characteristics of the scanning magnet of the implanter. Thus an ion beam having a round cross section of a few centimeters in diameter is formed at the output of the implanter.
Various internal modifications of the high-voltage terminal have allowed this implanter, with an initial implantation energy of 200 to 250 KeV, to be converted into a high-energy implantation machine (1 MeV for p4+ ions or even 1.5 MeV for p6+ ions). According to the inventors of the aforementioned implanter, it has thus been possible to implant with doses of 1014 ions per cm2, and this being achieved within a time compatible with production requirements (a 1014 dose obtained in two minutes for a wafer 100 mm in diameter). Finally, these operating conditions, which do not require the use of a hot filament, or moving parts, or of low pressure in the plasma chamber, considerably increase the lifetime of the source compared to that of hot-filament sources.
However, in spite of all advantages, the ion source used in the implanter of U.S. Pat. No. 5,625,195 possesses a number of disadvantages. In particular, the ion beam generated by this source, which has a round cross section of a few centimeters at the source output, has an energy of about 20-25 KeV. For further acceleration of ions to the level of energy required for implantation, the implanter that utilizes this source requires the use of an expensive and complicated ion-accelerating system, and without this system the implanter cannot develop beam energies sufficient for effective implantation. Furthermore, the ion source of U.S. Pat. No. 5,625,195 does not ensure uniformity of the ion beam current over the entire cross section of the beam extracted directly from the ECR plasma source. Another disadvantage of the known ion source is that it does not allow for adjustment of ion beam current distribution at the input to the magnetic separator and beam accelerator.
The applicants of the present patent application made an attempt to solve the problems of the prior art by developing an improved ion source for use in conjunction with an ion implanter, which is described in pending U.S. patent application Ser. No. 09/476,529 filed on Jan. 3, 2000. This patent application describes an ion source for implanting charged ions, e.g., of B++, P++, or the like, accelerated to the energy of a few hundred KeV. This ion source is characterized by radial direction of plasma extraction. The device is provided with a confinement space formed within a sealed vacuum chamber inside the housing of the implanter. The ions are extracted by means of a trans-axial electrostatic ion lens having a profile conforming to the boundaries of the plasma. The ion beam is then expanded by a second lens, which emits a substantially parallel ion beam of a rectangular cross-section onto the surface of the object being treated, which is moved across the ion beam. The profile of the plasma boundaries in the confinement space is determined by currents in a plurality of magnetic coils arranged in a number of horizontal layers around the plasma confinement space. If necessary, the profile of the plasma could be adjusted by measuring the ion beam current density distribution with sensors, such as Faraday cylinders, and then adjusting the currents in the aforementioned coils via a feedback mechanism.
The ion-beam source of U.S. patent application Ser. No. 09/476,529 will now be described in more detail with reference to the most essential parts and their operation. FIG. 1 is a longitudinal sectional view of the aforementioned ion beam.
The ion source, which as a whole is designated by reference numeral 20, is an ECR plasma source. ECR plasma source 20 has a housing 22 which is composed of two concentric cylindrical bodies, i.e., an outer cylindrical body 24, which is made of a non-magnetic material such as a stainless steel and is grounded at G1, and an inner cylindrical body 26, which functions as an anode, which also is made of a non-magnetic material such as a stainless steel. A positive potential, e.g., 80 kV, is applied to the inner body or anode 26 from a DC power source 28 via a conductor 30 and a high-voltage high-vacuum feedthrough 31. Such feedthroughs are standard devices, produced e.g., by Ceramsel Co., N.Y. and are intended to supply electric current to internal units of high-vacuum systems without violation of vacuum conditions. Outer cylindrical body 24 and anode 26 are interconnected by means of insulating spacers 32 and 34 to form an integral unit.
A cylindrical space 36, sufficient for placing magnetic coils described below, is formed between outer cylindrical body 24 and anode 26. Housing 22 is closed from both ends by covers 38 and 40 via sealing devices 42 and 44 so that the interior of housing 22 is sealed.
A plasma-confining magnetic system of ECR ion source 20 is defined by a plurality, e.g., four or six, of diametrically opposite paired magnets. Only two rows of such geometrically opposite magnets of these pairs, i.e., 46a, 46b, . . . 46n on one side and 48a, 48b, . . . 48n on the opposite side are shown in FIG. 1. An inner cavity 50 of anode 26 functions as a plasma-confining cavity. The plasma generated in this plasma-confining cavity is shown in FIG. 1 as a shaded area designated by reference numeral 52.
Magnets 46a, 46b, . . . 46n and 48a, 48b, . . . 48n are designed for confining plasma 52 in the inward radial direction in plasma-confining cavity 50, thus compacting it away from the inner walls of cylindrical anode 26. In order to confine the plasma in cavity 50 from end faces of this cavity, plasma source 20 is equipped with annular magnetic coils 54 and 56 arranged on opposite ends of housing 22.
Inner cavity 50 is connected to a source of vacuum (not shown) via an evacuation port (not shown) formed in lower cover 40.
As has been mentioned earlier, cylindrical space 36 is formed between outer cylindrical body 24 and anode 26. This space is necessary to install several pairs of magnetic coil arrays. Two such arrays 47 and 49 are shown in FIG. 1.
A trans-axial lens unit 58 (FIG. 1) is formed in the wall of anode 26 and projects radially outwardly from housing 22 of the ion source. Trans-axial lens unit 58 extends in the longitudinal direction of housing 22 almost along the entire length of the housing. Trans-axial lens 58 consists of three hollow electrodes 60, 62, and 64 located one inside the other with spaces 66 and 68, respectively, between the adjacent electrodes. In other words, space 66 is formed between the inner wall of electrode 60 and the outer wall of electrode 62, whereas space 68 is formed between the inner wall of electrode 62 and the outer wall of electrode 64.
Hollow electrode 60, which is the outermost electrode of the package, is supported by cylindrical anode 26 and is in electric contact therewith. As has been mentioned above, a potential of 80 kV is applied to cylindrical anode 30 from power source 32. Therefore the same potential will be applied to hollow electrode 60. A distal end of trans-axial lens 58 is open into plasma-confining cavity 50 in the form of a narrow ion-extracting slit of the same geometry as slit 68 shown in FIG. 4 of our previous patent application No. 09/476,529. This slit has a special profile described in the aforementioned patent application.
Innermost hollow electrode 64 has the same configuration as outermost electrode 60. Hollow electrode 64 is grounded. Electrode 64 has an ion extraction slit on its distal end and an ion outlet opening 70 on the outer or proximal end. The construction of the electrodes 60, 62, 64 and their slits are the same as in our previous patent application.
Located between outermost electrode 60 and innermost electrode 64, is intermediate hollow electrode 62. A negative potential, e.g., xe2x88x923 to xe2x88x925 kV, is applied to intermediate electrode 62 from a negative terminal of an electric power source (not shown). Intermediate electrode 62 is electrically insulated from innermost electrode 64 and outermost electrode 60.
Reference numeral 72 designates a second ion optical lens, which may be installed inside a hollow ion-beam guide 74 which extends further in the direction of propagation of the ion beam extracted from the plasma source 20. Similar to trans-axial ion lens 58, lens 72 is formed by hollow electrodes; in this case by two hollow electrodes 76 and 78 located one inside the other with a space 80 between them. This ion beam lens has a convex profile on the side facing trans-axial lens 58. Electrodes 76 and 78 have slits (not shown), which are aligned with each other and with the slits of trans-axial lens 58.
Thus, trans-axial lens 60 and ion beam lens 72 in combination form a telescopic ion beam system which may form an ion beam of a rectangular cross-section or a strip-like substantially parallel ion beam, i.e., an ion beam with very small angles of divergence in mutually perpendicular planes, i.e., in the plane of FIG. 1 and in the plane perpendicular thereto.
In FIG. 1, reference numeral 82 designates a waveguide for transmitting microwave energy with the frequency, e.g., of 2.45 GHz, required for creating so-called electron-cyclotron resonance (ECR) conditions described in our previous patent application. Waveguide 82 comprises a hollow metallic tube 84 of a rectangular cross-section made of a highly conductive material, e.g., silver-coated copper. Tube 84 is connected to cylindrical anode 26 and is electrically isolated therefrom by means of a sealing device 86. An outlet opening 88 of tube 84 into plasma-confining cavity 50 is closed by a heat-resistant window 90 transparent to microwave energy. An example of such a material is quartz. The interior of outer cylindrical body 24, i.e., plasma-confining cavity 50, as well as space 36, and the entire inner cavity 50 of ion-beam source 20 are sealed from the environment surrounding ion-beam source 20.
A working medium, e.g., a boron-containing gas such as BCl3, BF3, or a phosphorus-containing gas such as PH3, etc., is supplied into interior cylindrical anode 26 via a tube 92 which passes into this interior through a standard high-vacuum, high-voltage resistant feedthrough device 94. Such a feedthrough device is produced, e.g., by Insulator Seal Incorporated, Hayward, Calif., USA.
In order to enhance the energy of plasma, ion-beam source 20 is equipped with at least one antenna for supplying a radio-frequency (RF) power into the plasma 52. In the embodiment, illustrated in FIG. 1, this device has two such antennas 96 and 98, which can be inserted into plasma-confining cavity 50, e.g., through magnets 54 and 56, although the antennas can be inserted through any other locations. It is understood that antennas 96 and 98 should be inserted into cavity 50 without violation of vacuum conditions, i.e., through appropriate high-vacuum, high-voltage resistant feedthrough devices 100 and 102 of the same type as those mentioned above. Terminals 104 and 106 located on outer ends of antennas 96 and 98 are connected to appropriate microwave sources (not shown), e.g., of 13.72 MHz frequency.
Operation of ion source 20 will be further described with reference to aforementioned FIG. 1. Plasma-confining cavity 50 of ion source 20 is evacuated via the evacuation port by means of a vacuum pump (not shown). Microwave energy of 2.45 GHz is pumped into cavity 50 inside hollow anode 26 (a MW generator is not shown). When vacuum reaches a predetermined level, e.g., of 0.5 mTorr, a working medium, e.g., a boron-containing gas, is supplied via gas supply tube 92 into cavity 50. The plasma-confining magnetic system formed by the magnet arrays 46, 48, etc., generates plasma magnetizing and confining magnetic fields inside cavity 50.
In some areas of cavity 50, magnet arrays 46, 48, etc. generate magnetic fields within a strength of 0.0875 Tesla, which is a resonance field for 2.45 GHz frequency oscillation of electrons. As a result, these electrons begin to intensively consume the microwave energy. This phenomenon, which is known as an electron cyclotron resonance (ECR), enhances plasma and allows the development of plasma charge densities of up to 1013 e/cm3. In other words, a very dense plasma 52 is developed in the cavity 50. Plasma 52 is further intensified by radio frequency supplied into cavity 50 via antennas 96 and 98.
For effective extraction of plasma 52 from plasma-confining cavity 50, it is necessary that the outer plasma boundary conform to the profile of the trans-axial lens 60 on its distal end, where plasma-extracting slits are formed. This is achieved by means of the aforementioned arrays 47 and 49 of magnetic coils. Since the coils of these arrays have their own individual power sources (not shown), the magnetic fields developed by these coils can be individually adjusted to ensure fine conformity of the plasma boundary to the lens profile. After such conformity is achieved, positive boron ions are extracted from plasma 52 via the plasma emitting slits of trans-axial lens 60. Due to the fact that the boron ions are double-charged (B++) and that above-described potential difference between three outermost hollow electrodes 60 and innermost hollow electrode 64 of trans-axial lens 60 is about 85 kV, boron ions may develop in the interelectrode magnetic fields energies of about 170 KeV. An ion beam IB formed on the output of trans-axial lens unit 58 is diverged (FIG. 1), and when it passes through ion lens 72, its angle of divergence is reduced, so that an almost parallel ion beam of a rectangular cross section exits ion-beam guide 74 and enters a working vacuum chamber (not shown).
In spite of the advantages inherent in the ion-beam source of U.S. patent application Ser. No. 09/476,529 filed on Jan. 3, 2000, it still possesses some drawbacks. In particular, the aforementioned ion-beam sources can produce ions only from gaseous working materials. In other words, material to be implanted is supplied to the plasma chamber only in a gaseous phase. Furthermore, when the aforementioned source generates a belt-like ion beam of a rectangular cross section, which is to be delivered to the treated object through the output of the implanter, the longer dimension of the aforementioned rectangular cross section, which hereinafter will be referred to as a width of the ion beam, is limited substantially to the length of the microwave pumping waveguide. For microwave energy pumping, e.g., of 2.45 GHz, such a waveguide cannot have an ion beam width exceeding 15-20 cm, even with waveguide output cross-section modified for obtaining the maximum dimension. This, in turn, limits efficiency of the source.
As mentioned above, the interior of vacuum chamber 50 of ion source 20, which normally operates under conditions of deep vacuum at about 10xe2x88x928 Torr or lower is sealed from the microwave pumping system by quartz or ceramic windows 90 transparent to microwave energy. During operation of ion source 20, these windows are contaminated by plasma particles from the side of plasma chamber 50. Contamination of the windows may reach such a level that further use of the source becomes impossible because of non-transparency of windows 90, which in this case do not pass microwave energy to cavity 50. This violates plasma-sustaining conditions. Therefore, when the windows are contaminated, the entire system has to be stopped, the source has to be disassembled and the windows have to be cleaned or replaced. This disadvantage is reflected in increased costs of production and maintenance.
Another specific disadvantage inherent in the ECR ion source described in the aforementioned U.S. patent application Ser. No. 09/476,529 consists in that radial extraction of ions is carried out with the use of a trans-axial three-electrode ion lens. Although the aforementioned trans-axial three-electrode ion lens is advantageous in that it provides an extremely high uniformity of distribution of ions in the narrow beam produced by this lens, the drawback of this lens is the use of three electrodes. This is because these electrodes work under conditions of significant potential difference with respect to each other. Such a mode results in high current losses which lead to decrease in the efficiency of the ion source as a whole.
It is an object of the invention to provide an ion-beam source for use in an ion implanter which is suitable for operation with gaseous as well as with solid materials for generation of ions. Another object is to provide an ion-beam source of the aforementioned type having an increased width of the ion beam, which may exceed 20 cm. Still another object is to provide an ion-beam source for use in an ion implanter with a mechanism for periodic or continuous cleaning of waveguide output windows. Further object is to provide an ion-beam source for an ion implanter, which is simple in construction because it is free of a trans-axial three-electrode lens, reliable and efficient in operation, and inexpensive to manufacture. Still another object is to provide a method for generation of ions from gaseous and solid materials in efficient way and in the form of wide ion beams.